MIPS: start.S: make boot config at offset 0x10 configurable

Some MIPS systems store some board-specific boot configuration
in the U-Boot binary at offset 0x10. This is used by Malta boards
and by Lantiq/Intel SoC's when booting from parallel NOR flash.
Convert the hard-coded values to Kconfig options to remove such
board-specific stuff out of the generic start.S code. This also
deprecates the config option CONFIG_SYS_XWAY_EBU_BOOTCFG.
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>